In normal vehicles, a kingpin axis defines or sets a kingpin angle (inclination of the kingpin axis relative to the vertical direction when the kingpin axis is seen from the front side of the vehicle) and a caster angle (inclination of the kingpin axis relative to the vertical direction when the kingpin axis is seen from the lateral side of the vehicle), and a camber angle which is formed when the wheel is steered (angle defined by the longitudinal center line of the wheel and the vertical line when the wheel is seen from the front side of the vehicle) increases the stability of the vehicle traveling in a straight line. However, as illustrated in FIGS. 13(a) and 13(b), a camber angle δ resulting from setting a kingpin angle keeps a wheel w of a vehicle inclined toward one side of the wheel in its width direction (i.e., keeps the wheel w in the “negative camber” state illustrated in FIG. 13(a) or in the “positive camber” state illustrated in FIG. 13(b)), so that the wheel is likely to interfere with the body of the vehicle or a component of the vehicle such as a stabilizer.
The larger the steering angle of the wheel w is, the larger the camber angle δ tends to be. Therefore, in order to prevent the interference between the wheel w and the body of the vehicle, etc., it is necessary to limit this steering angle within a predetermined range or to enlarge a tire house. As a result thereof, the turning radius is made large, and thus the vehicle is unable to turn with a small turning circle. Moreover, it is difficult to make the vehicle small-sized.
Moreover, uneven wear is likely to arise in the wheel w due to the inclination of the wheel w toward one side of the wheel in its width direction, and a large steering force is necessary to steer the wheel w due to the upward and downward motions of the vehicle, which result from the variation of the surface area of the vehicle w coming into contact with the ground, thus deteriorating steering operability.
In order to overcome such problems, in the below-identified Patent document 1, at least one of a kingpin angle (see “θ” in FIG. 1 of the present application) and a caster trail (the distance between the point of a kingpin axis coming into contact with the ground and the point of the center axis of a tire coming into contact with the ground when seen from the lateral side of a vehicle (see “c” in FIG. 3 of the present application)) is set at substantially 0 (zero) degree (in case of the kingpin angle) or 0 (zero) mm (in case of the caster trail), thereby preventing a camber angle δ from being formed when a wheel is steered. Moreover, a vehicle behavior detecting means, and an electric steering device are provided, and the motion and steering performance of the vehicle are increased by applying steering torque.